This invention relates to hearing aids and more particularly to a method and means for enabling the user of a hearing aid to hear speech from a remote distance even though the level of the speech may be lower than the ambient noise level in the region of the user.
A conventional hearing aid of the prior art is worn in or about the user's ear or on the body and comprises a housing having therein a microphone for receiving audible sound and converting it to an electrical signal, and an amplifier for amplifying the electrical signal. A transducer, which may or may not be within the housing, converts the electrical signal back to audible sound which is then applied to the user's ear.
A problem arises when a hard-of-hearing person attempts to use a hearing aid in an environment having a high ambient noise level. The conventional hearing aid amplifies all sound picked up by the microphone to the same degree, hence ambient noise is amplified and may override a voice communication. Consider, for example, a typical speech training classroom wherein the ambient noise level may exceed 70 db. The voice level of a teacher, at a distance of six inches from the mouth, is typically 80 db. However, since sound power drops 6 db for each doubling of distance, within a very few feet a negative ambient noise to teacher voice levels exists. Meaningful communication is not possible since the noise is amplified by the same factor as the teacher's voice. Thus, it is necessary to establish a voice link between the teacher and student.
One of the currently used methods of establishing the teacher-student voice link involves FM transmission of the teacher's voice to the student's receiver. In addition to receiving the FM transmission from the teacher, the student's receiver is provided with one or two microphones for reception of his own voice (an important requirement for speech training) and the voices of other students in the class. Because of the radio link, the teacher's voice is always clearest because the level of the teacher's voice is set to dominate over any environmental noise.
A further disadvantage exists in the prior art system in that it requires a separate receiver/aid and the student's personal aid must be removed during class hours. Thus, the prior art system requires a duplication of equipment and further requires that the student's personal hearing aid, that has been carefully adjusted by an audiologist, must be removed and replaced by a completely different unit that may not have the same characteristics as the personal aid.
Some attempts have been made to avoid this duplication and enable the student's personal hearing aid to be used in the classroom. These attempts have employed a magnetic (telephone pickup) coil in the aid to couple in the voice from a radio link. This arrangement suffers from several disadvantages. Switching to the "tele" position disconnects the internal microphone and prevents the student from hearing his own voice. The "tele" coils vary widely in performance and do not give the consistency of frequency response required for proper speech development. External magnetic fields may seriously interfere with proper reception. Finally, many personal hearing aids do not include a "tele" coil, thus preventing their use in a classroom system.